In general, a power steering system for an automotive type vehicle includes a control shaft or steering column which is controlled by the vehicle operator turning the steering wheel, and a power assist device for the driven steering system. Hydraulic devices are conventionally used to provide the power assist. In addition, it is known to use a servo-motor arrangement to provide the power assist for the drive. An example of the use of a servomotor arrangement is disclosed in U.S. Pat. No. 3,893,534 issued July 8, 1975 to Steinmann.
In any power assist device used in a power steering system, there is a danger of malfunctioning or power failure. Although failures are rare, it is necessary to provide a direct drive alternative to the power assist drive during times of malfunctioning or failure. When the servo-motor resists or opposes the action of the driver, the servo-motor should be disengaged, whereas it should remain engaged if it provides some assistance even though it may be in a state of partial failure. Before my invention set forth in U.S. Ser. No. 392,990, systems lacked these qualities to some degree or had other deficiencies which kept them from achieving commercial success.
In addition to the need for the fail-safe feature mentioned above it is desirable to minimize the resistance or drag imposed by the drive train of the servo-motor on the movement of the dirigible wheels to the straight-ahead position when the steering wheel is allowed to return to the straightahead position.
A general object of this invention is to provide a coupling for power steering which provides the aforementioned features and overcomes certain disadvantages of the prior art.